Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel and more efficient than portable power storage, such as lithium-ion batteries.
In general, fuel cell technologies include a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into three general categories, namely fuel cells utilizing compressed hydrogen (H2) as fuel, proton exchange membrane or polymer electrolyte membrane (PEM) fuel cells that use methanol (CH3OH), sodium borohydride (NaBH4), hydrocarbons (such as butane) or other fuels reformed into hydrogen fuel, and PEM fuel cells that use methanol (CH3OH) fuel directly (“direct methanol fuel cells” or DMFC). Compressed hydrogen is generally kept under high pressure, and is therefore difficult to handle. Furthermore, large storage tanks are typically required, and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. DMFC, where methanol is reacted directly with oxidant in the fuel cell, is the simplest and potentially smallest fuel cell, and also has promising power application for consumer electronic devices.
DMFC for relatively larger applications typically comprises a fan or compressor to supply an oxidant, typically air or oxygen, to the cathode electrode, a pump to supply a water/methanol mixture to the anode electrode and a membrane electrode assembly (MEA). The MEA typically includes a cathode, a PEM and an anode. During operation, the water/methanol liquid fuel mixture is supplied directly to the anode, and the oxidant is supplied to the cathode. The chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Half reaction at the cathode:O2+4H++4e−→2H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode through the cathode and due to the inability of the free electrons (e−) to pass through the PEM, the electrons must flow through an external circuit, which produces an electrical current through the external circuit. The external circuit may be any useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants and laptop computers, among others. DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfonic acid polymer having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes.
The electrochemical cell reactions take place at a membrane electrode assembly typically comprised of an anode diffusion layer, comprised of a carbon paper support treated with a fluoropolymer, such as Teflon® available from DuPont, an anode catalyst layer comprised of catalyst, such as platinum-ruthenium, and a proton conductor, such as Nafion® perfluorinated sulfonic acid polymer, the PEM, a cathode catalyst layer comprised of catalyst, such as platinum and a proton conductor and a cathode diffusion layer comprised of a carbon paper support treated with a fluoropolymer.
The cell reaction for a sodium borohydride reformer fuel cell is as follows:NaBH4(aqueous)+2H2O→(heat or catalyst)→4(H2)+(NaBO2)(aqueous)H2→2H++2e− (at the anode)2(2H++2e−)+O2→2H2O (at the cathode)Suitable catalysts include platinum and ruthenium, among other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. Sodium borate (NaBO2) byproduct is also produced by the reforming process. Sodium borohydride fuel cell is discussed in United States published patent application no. 2003/0082427, which is incorporated herein by reference.
Cations other than protons reduce the conductivity of the PEM. Especially damaging to the membrane conductivity are multivalent metal ions which tend to get trapped in the PEM. When the conductivity is sufficiently reduced or when the level of trapped ions reaches a threshold level, the PEM has to be replaced or refurbished.
The patent literature discloses a number of filters for fuel cells. U.S. Pat. No. 6,265,093 B1 discloses a direct methanol feed fuel cell system that includes a fuel filter located in front the MEA. This filter is a sieve-type filter that traps particles based on particle size to remove hydrocarbon impurities from the fuel. U.S. Pat. No. 6,630,518 B1 discloses a polymer membrane that is irradiated and then sulfonated to link the sulfonic acid group to the membrane. The membrane is usable as the PEM in fuel cells and is usable as ion-exchange member or in ion-selective purification systems, among other uses.
Therefore, a need exists for a filter that reduces metal ion concentration in fuel for use in a fuel cell.